I agree with the advice from the others. I have one cat who is quite large (not fat just a big boy) and he sometimes prefers to use our bathtub or the mat right next to the box. I believe it is because he is quite large and finds it uncomfortable to balance in the box while pooping
. I personally find it fairly easy to deal with the poop on the mat, not really any different from scooping it out of the litter box. I think making a big deal of it may force him to hide elsewhere to do his business and that would be much more of a problem.
BTW, I had to laugh at "he must learn to obey the rules". Didn't you know that cats are the ones who set the rules
.